1. Field of the Invention
The present invention relates generally to RF devices, and particularly to broadband coupler and balun devices.
2. Technical Background
An unbalanced line typically includes a signal line conductor and a grounded conductor. The ground conductor may form a ground plane. An unbalanced line is typically employed in the RF portion of the signal path (i.e., between the antenna and the receiver). The signal is typically demodulated from an RF frequency to an intermediate frequency (IF) that is more suitable for the receiver. At this point in the signal path, the use of differential signaling is being considered as a way to improve the receiver performance; and differential signals require a balanced line. Of course, those skilled in the art will appreciate that differential signaling is also used in RF signal paths (i.e., not only in the IF portion of the system). For example, certain antennas are balanced structures that require a balanced feed, and a push-pull power amplifier may be used to provide the differential RF signal.
A balanced line, or balanced signal pair, is a transmission line consisting of two conductors of the same type, each of which have equal impedances along their lengths and equal impedances to ground and to other circuits. In a communications device, such as a cell phone, balanced transmission lines can be used to transmit a differential signal pair; i.e., a pair of signals that is equal in amplitude but opposite in phase (i.e., 180° out of phase). Thus, the sum of the two signals that comprise a differential signal pair is zero. Moreover, when a noise signal is introduced onto a balanced line (the differential transmission path), that noise signal propagates on both signal paths. (Noise of this type is known as common-mode noise since it propagates on both signal paths). Thus, when the two signals that comprise the differential signal pair (plus noise) are subtracted from each other, the noise signal is cancelled and the pair of signals is added together. Thus, differential signals have inherent noise cancellation properties.
A balun is a component that functions as an interface between a balanced transmission line and an unbalanced transmission line. (The word “balun” is shorthand for two words, balanced and unbalanced). Thus, a balun can be used to combine the differential signal pair to form an unbalanced signal; or split an unbalanced signal (i.e., a single ended signal) into a differential signal pair. In addition, the balun may perform impedance conversion between the unbalanced system and the balanced system. Thus, a balun can play a very important role in the system design.
In a push-pull power amplifier application, for example, a first balun can be used at the input stage to split the single ended signal. After amplification, a second balun can be used to combine the differential signals at the output stage.
Consider the following example wherein a 50 Volt DC biased LDMOS transistor is required to provide a 100 watt output. This configuration typically requires the load impedance to match to around 6.25 Ohm. What is needed is a balun with 6.25 Ohm impedance at each of the differential ports, and 50 Ohm impedance at the single ended port. A balun such as this would essentially eliminate the need for a power amplifier output matching network. In general, what is needed in a push-pull power amplifier application, therefore, is a balun that can convert a 50 Ohm single ended port impedance to a specified lower differential port impedance. A balun that can achieve the aforementioned design objective would simplify the design of the power amplifier's input and output matching networks.
One important parameter of a differential amplifier relates to its ability to reject common mode noise. To be specific, this parameter is appropriately known as the common-mode rejection ratio (CMRR); it is the ratio of the differential signal (desired) to the common mode noise (undesirable). Thus, a device that featured an infinite CMRR would be ideal. Stated differently, CMRR quantifies how well the balun rejects common mode signal. In push-pull power amplifier applications, the CMRR is directly related to power amplifier's second order harmonic cancellation. To achieve high linearity and efficiency in push-pull power amplifier, it is desired to have a balun with a good wideband CMRR.
In one approach that has been considered, a balun was realized by winding coaxial cables through ferrite cores. This approach achieved an operating frequency from DC-GHz range. The ferrite core enabled the balun to achieve relatively good performance at low frequency, especially in the frequency range of <100 MHz. However, one drawback to this approach relates to the fact that the insertion losses are significantly increased at higher frequency (>1 GHz) due to the nature of the ferrite core. In a similar approach that employed ferrite beads, the insertion loss of a wire wound balun was approximately 0.8 dB at 900 MHz. Another drawback to this approach relates to the manufacturability of this type of design. For example, an assembly procedure for a balun comprising ferrite beads and coaxial cable requires intensive labor processes. Thus, this approach is expensive and not amenable to cost reduction; moreover, the non-standardized nature of human assembly tends to introduce performance variations.
To improve the manufacturability and the high frequency performance, balun can also be implemented in surface mount packages using printed circuit technology. The manufacturing tolerance control in PCB technology is much more consistent than coaxial balun's assembly process. However, the elimination of coaxial cables and ferrites in the balun design comes with the cost of the significant decrease in even mode impedances.
Referring to FIG. 1, a schematic diagram of a conventional inverted Guanella balun is shown. The Guanella balun 1 is configured to convert an unbalanced port impedance of Zs to a balanced port impedance of Zs/4. The Guanella balun 1 includes two quarter wavelength couplers (2, 3) that are characterized by an even mode impedance of Ze and odd mode impedance of Zo; wherein Zo equals the balanced port impedance Zs/4. One drawback to this approach is that the even mode impedance of Ze limits the bandwidth of the CMRR. Moreover, an insertion loss null may occur around the center frequency in the band due to variations in manufacturing processes.
In another approach, an inverted Marchand balun has been considered. One benefit to this approach is that, unlike the Guanella balun discussed above, it may not require a large even mode impedance (to obtain a reasonable CMRR bandwidth). However, one drawback to this approach relates to the fact that the return loss and insertion loss bandwidths are very limited for the impedance transforming ratio of 4 to 1 and above.
What is needed, therefore, is a RF balun device that addresses the concerns highlighted above. To be specific, what is desirable is a RF balun device that broadens the achievable CMRR and return loss bandwidths. What is also needed is a balun that employs planar printed circuit technology, yet at the same time, provides performance that is comparable to a ferrite coaxial balun.